1. Field of the Invention
An object of the present invention is a permanent magnet actuator with electric excitation coil, especially a mobile telephone loudspeaker or, again, a personal microcomputer loudspeaker. The aim of the invention is to make the motion of such an actuator insensitive to external magnetic disturbances.
2. Description of the Prior Art
Mobile telephones use loudspeakers that are naturally placed towards the top of the machine, so as to correspond to the position of a listener's ear. The electromagnetic radiation antenna of a mobile telephone such as this is also placed towards the top of the instrument. In practice, an electric printed circuit forming an electronic board is placed so as to be facing the loudspeaker and conveys transmission signals up to this antenna. When the mobile telephone is put into service and recognized by a base station, this mobile telephone sends out protocol signals, with rated power, on the beacon frequency of thus base station, before this base station assigns it a lower value of transmission power, related to its distance. When the mobile telephone is far from the base station, the transmission power is furthermore permanently a rated power value.
The electrical signal corresponding to this protocol-based acknowledgement, like all the signals exchanged, are contained in 577-microsecond time windows in eight-window frames whose duration is 4.615 milliseconds (in GSM TDMA—Time Division Multiple Access—mode). The output amplifier of the mobile telephone is therefore the site of a pulsed consumption with a frequency of the order of 217 Hz. This high consumption generates an electromagnetic noise that gets propagated in the pack of the mobile telephone and, in certain cases, leads to a parasitic mechanical excitation of the diaphragm of the mobile telephone loudspeaker. The user then hears a rumbling noise, at least at the time of recognition. This noise is troublesome.
The disturbance observed is not a disturbance of the audio signal that is fed into the loudspeaker (even if this source also plays a part), but an induction of current in the coil of the loudspeaker. This induction of current has the effect of placing the coil and diaphragm of the speaker into motion and therefore of producing noise. This noise is called burst noise with reference to the pulse envelopes of the power values consumed in the temporal windows. Thus, when the loudspeaker is short-circuited on itself, without any electrical contact with the printed circuit, the burst noise is perceived in the same way. In certain mobile telephones, the problem is less critical because the loudspeaker is relatively distant from the printed circuit on the electronic card. Inasmuch as it is difficult to foresee the structure of the magnetic field radiated by an electronic board, this type of problem can be detected only when the loudspeaker is integrated into the mobile unit, even if it is known that it is always preferable to move the loudspeaker away to the maximum distance from the high-power radio tracks.
In normal operation, a voltage U(t) is applied to the terminals of the loudspeaker. The excitation coil of this loudspeaker is then crossed by a current i. This coil is furthermore immersed in a static magnetic field B0 created by a magnet of the loudspeaker. The result thereof is a force F applied to the coil, called the Laplace force, whose value is given by: F=ilB0, l being the wire length of the coil. This force is sometimes positive and sometimes negative depending on the direction in which the current travels in the coil. Since the coil is rigidly fixed to the diaphragm of the loudspeaker, the assembly formed by the coil plus the diaphragm is thus put into motion.
In disturbed mode, the excitation voltage originates differently. An external electromagnetic field Be(t), variable in time, encompasses the coil of the loudspeaker and creates a magnetic flux Øe through this coil. The value of this flux is given by Øe(t)=NSBe(t), where N is the number of turns of the coil and S is the surface area of a turn. It is the variations in time of this flux that generate an electromotive force (a voltage) e at the terminals of the coil according to the relationship e=−dØe/dt. The voltage e faces a relatively low electrical resistance (the electrical resistance of the loudspeaker (R=8 Ω) in series with the internal resistance of the amplifier (r≈8 Ω)). Hence a non-negligible current crosses the coil. This activates the diaphragm according to the same principle as the one referred to here above. An approach in which a high resistance is series-connected with the loudspeaker has the merit of eliminating the perceptible effects of such a noise. However, it has the drawback of leading to an audio supply that is oversized and consumes power.
It is an object of the invention to overcome the above-mentioned drawback by combating one ill with another ill. It is known indeed that a voltage +e appears at the terminals of the coil when it is plunged into the disturbing field Be(t). In the invention, a second coil, identical to the first one, is then placed in the vicinity of the first one, resulting in the appearance, in the same way, of a voltage +e at the terminals of this second coil What remains to be done then is to achieve a series-connection of the two coils, but in opposition (namely with opposite directions of winding), so that the two voltages get cancelled out. If, because of the disturbance Be(t), no voltage is applied to the terminals of the two coils, then no corresponding current flows therein. If no current flows therein, then no mechanical force is exerted on the coil and, therefore, because of the disturbance, the diaphragm of the loudspeaker does not move.